I'd think that a pneumatic bottle would have to be very large, and/or very heavy to store enough energy for a useful return, the heat lost from the compressed a

I'd think that a pneumatic bottle would have to be very large, and/or very heavy to store enough energy for a useful return, the heat lost from the compressed air would lower efficiency, and an air turbine that can provide usable power over a wide range of pressures from the bottle would add additional weight and contribute to inefficiency. The fact that there are few compressed air hybrids is probably a good sign that it's not significantly better than batteries. Few electric bikes even use regenerative braking, it's generally not worth the weight and complexity.